The invention relates to tape tension control circuits and particularly to a digital open loop servo circuit for automatically controlling the tension on a tape without need for a tension sensing transducer.
Various closed loop tension control circuits are available, for example, in the tape recorder/reproducer art, wherein tension sensors are used to detect instantaneous tape tension. A corresponding feedback signal then is supplied to regulate the torque of the associated motor, or to adjust a tape tension arm, to maintain a preselected tape tension. Typical of such closed loop systems are those described in, for example, U.S. Pat. No. 3,535,441 to A. G. Grace and U.S. Pat. No. 3,936,008 to J. N. Crum.
In addition, various open loop tension control circuits also are available, employing analog circuits, which preclude the need for tape tension sensors. Typical of such an open loop system is that employed in the tension accessory control apparatus in the Ampex Corporation AG-440 professional audio recorder and described in the tension accessory control Model TS-40 Manual, catalog No. 4890404. Further typical of such analog, open loop tension systems are those described, for example, in U.S. Pat. No. 3,871,598 to H. Kataoka; U.S. Pat. Nos. 4,084,768 and 4,090,679 both to K. Hayashi, etc.
Thus, in situations where tape tension sensors are not available, or where it would be undesirable to include tension sensors, it is preferable to use an open loop servo system to control tape tension. However, open loop analog circuits are prone to drift and are difficult to match to motor characteristics. In addition, integrated circuits such as, for example, linear operational amplifiers, are not readily available which can operate at the higher voltages found in many tape recorder, etc., apparatus.
Thus, digital open loop tension control circuits have been provided with their inherent stability advantages in systems wherein tape tension transducers are not available or desired. Typical of such digital tension control circuits are those described, for example, in U.S. Pat. No. 3,949,244 to A. M. Goldschmidt; U.S. Pat. No. 3,982,160 to A. M. Goldschmidt et al; etc. However, the latter digital circuits are generally complex, require additional input signals and do not provide D/A converter means which readily can be matched to the motor characteristics. Thus the circuits do not provide the precision of tension control which is possible in the present invention.